736 General and Applied Biology 



it is once started. The germ cell cycle is considered in Chapter 34. Two 

 of the more important phases of the process (Fig. 351) are (1) the pro- 

 duction of germ cells and (2) the maturation of the germ cells. 



IV. DEVELOPMENT OF LIVING ORGANISMS 



Development may be defined as the bringing to the fore what is al- 

 ready present but latent in an organism. The amount and type of devel- 

 opment of a particular organism depend on ( 1 ) its particular inheritance 

 and (2) the external and internal environmental factors which surround 

 it and in which it must develop. All living organisms are affected by 

 such factors as the quantity and quality of the food, amount of water 

 available, quantity and quality of light, the improper elimination of their 

 waste materials, their type of activity, the presence or absence of vita- 

 mins, enzymes, etc. 



V. DESCENT OF ORGANISMS WITH CHANGE 

 (EVOLUTION) 



A. Evidences of Descent With Change 



Evidences of descent with change have been secured from such sciences 

 as (1) paleontology (science of fossil remains), (2) taxonomy (classifica- 

 tion), (3) comparative embryology, (4) comparative anatomy, (5) com- 

 parative physiology, (6) biogeography (geographic distribution), and 

 (7) genetics and variations. 



1. Evidences From Paleontology. — Geologists can determine, in most 

 cases with remarkable accuracy, the chronologic succession in time of 

 the various strata composing the earth. The fossils of these various 

 strata testify to the order of appearance and disappearance of various 

 types of animals and plants on the earth, the more recent appearing 

 nearer the surface. 



Two examples of descent with change may be cited. (1) Birds seem 

 to have evolved gradually from a rcptilelike ancestor, because, in spite 

 of superficial dissimilarities (such as the scaly-skinned, cold-blooded rep- 

 tile and the feathered, warm-blooded bird), there are many fundamental 

 structural and functional similarites not only between adult reptiles and 

 birds but also between their embryonic stages. Fossil remains of a rep- 

 tilelike bird (Archaeopteryx) show a connecting link between reptiles and 

 birds as they are known today (Fig. 362). (2) The horse also has de- 

 veloped to its present status through a series of successive changes. These 



